The use of renewable energy sources to generate electricity for the power network is becoming increasingly common in many countries. It is possible to convert renewable energy such as wind, wave, tidal energy or water current flows into electrical energy by using a turbine to drive the rotor of an alternating current (ac) electrical power generator, either directly or by means of a gearbox. The ac frequency that is developed at the stator terminals of the generator is directly proportional to the speed of rotation of the rotor. The voltage at the generator terminals also varies as a function of speed and, depending on the particular type of generator, on the flux level.
In some circumstances, it can be advantageous to transmit electrical power generated by a renewable energy turbine via a high-voltage direct current (HVDC) power transmission network, as opposed to a more conventional ac power transmission network. A dc source power converter in the form of a generator bridge and operating as an active rectifier connects the ac electrical power generator of the renewable energy turbine to the HVDC power transmission network. The renewable energy turbine and its associated ac electrical power generator and dc source power converter thus operate together as an individual dc source supplying dc electrical power to the HVDC power transmission network. It will be understood that a large number of such dc sources are typically connected in parallel to the HVDC power transmission network to supply the required amount of dc electrical power to the network and ensure stable network operation.
The individual dc sources can operate under voltage control regulation to supply electrical power at a target or reference voltage value Vref to the HVDC power transmission network and/or under current control regulation to supply electrical power at a target or reference current value Iref to the HVDC power transmission network, with a combination of voltage control regulation and current control regulation being more advantageous. During fault conditions, either in the HVDC power transmission network or in one or more of the parallel-connected dc sources, the output voltage at the converter terminals of one or more of the individual dc source power converters or the output current supplied by one or more of the individual dc source power converters can increase to levels that cannot be tolerated by the dc system. This can cause conflicts in the electrical power generated by the multiple parallel-connected dc sources.
There is, therefore, a need for a converter control arrangement and associated control methodology for a dc source power converter which can prevent conflicts in the electrical power generated by multiple dc sources operating in parallel, for example as a result of being parallel-connected to a dc system such as a HVDC power transmission network. More particularly, there is a need for a converter control arrangement and associated control methodology which is capable of providing reliable and effective regulation of the output voltage of a dc source power converter to minimise damage, either arising from excessive output voltage at the converter terminals or from excessive output current, especially during fault conditions.
US 2008/122412 A1 describes with reference to FIG. 4 a voltage regulator, specifically intended for a high-performance integrated circuit such as a microprocessor, which has a loadline with two different loadline resistances (i.e. droop rates). The voltage regulator controls the processor voltage so that its operating point is on a light loadline, with a lower loadline resistance, when the measured current is below a lower processor current level (IccLo) and so that its operating point is on a heavy loadline, with a higher loadline resistance, when the measured current is above the lower processor current level (IccLo). The operating point of the processor is not pre-defined by the voltage regulator and instead moves along the two fixed loadlines illustrated in FIG. 4 according to the load level (i.e. the measured current).